The platinum group metals, i.e., ruthenium, rhodium, palladium, osmium, iridium, and platinum (“PGM”), are often recovered from used catalyst materials such as, for example, automotive catalytic converters. The catalyst materials are smelted in a furnace, typically with a flux material such as CaO, and the PGM are preferentially collected in an alloy pool below the slag. While the PGM are dilute in the furnace slag, nevertheless these losses can be significant due to the high volume of slag and a general inability to economically recover the dilute values. The PGM collector alloys may contain up to 12 wt % PGM, and usually contain more than 40 wt % iron. Enrichment is necessary if a higher PGM content is desired.
PGM enrichment of iron-rich, sulfide-lean collector alloy by pyrometallurgical converting was disclosed in S. D. McCullough, “Pyrometallurgical iron removal from a PGM-containing alloy,” Third International Platinum Conference ‘Platinum in Transformation,’ The Southern African Institute of Mining and Metallurgy (2008). PGM enrichment of sulfur-free or low-sulfur (<1 wt %) PGM collector alloy was more recently proposed in patent documents U.S. Pat. No. 10,202,669 B2 and US 2018/0142330 A1. The PGM-enriched alloys generally contain a relatively high proportion of iron (>10 wt %).
There are a number of drawbacks associated with known converters and converting processes preventing them from being practically implemented to process PGM collector alloy generated from smelting catalyst materials. The converting process can be relatively slow. In the patent documents mentioned above, the collector alloy and slag-forming materials were melted for 10 hours prior to oxygen injection. Moreover, the converting process is exothermic, and the rate of oxygen addition is generally limited to avoid excessive temperatures. Further, the severe conditions in the converter, especially at high oxygen injection rates, lead to corrosion and short lifespans for refractory lining.
The industry has generally accepted that, similar to smelting, relatively high levels of added flux materials such as SiO2 and MgO/CaO are needed for the formation of a low melting, light density slag to adequately remove impurities and improve the PGM content of the PGM-enriched alloy product from a converter. For example, the aforementioned patent documents disclose the addition of sulfur- and copper-free slag-forming material in minimum proportions of 0.2 or 1 part by weight per 1 part by weight collector alloy, where the slag-forming materials contain 70-90 wt % SiO2 and 10-30 wt % MgO/CaO, or 40-90 wt % MgO/CaO and 10-60 wt % SiO2.
The industry needs technology that can address one or more of the shortcomings of conventional converting processes for PGM collector alloys. Such technology would desirably achieve one or more of the following: improve the alloy melting rate, oxygen addition rate, and/or the processing rate or capacity of the converter; provide lower levels of iron and/or deleterious materials in the PGM-enriched collector alloy; reduce PGM losses in converter slag; improve reliability and/or durability of converter components; reduce converter maintenance requirements and/or operating interruptions; and/or improve the efficiency and practicality of using converters incorporated as part of an overall process to recover and enrich PGM collector alloy, e.g., from catalyst or other PGM-containing materials.